There are 22680 numbers, excluding those with leading 0s.
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how many numbers exactly have 4 digits ? 8900, 8999, 9000, 9999
There are 90 of them.
Assuming that 001, 080, etc are not allowed (that is a leading zero or two is not permitted), the smallest number with exactly three digits is 100. The largest number with exactly three digits is 999. So there are 999 - 100 + 1 = 900 numbers with exactly three digits.
There are nine numbers which contain only one digit. There are 90 numbers which contain two digits. There are 900 numbers which contain three digits. There is one number which contains four digits.Therefore, the number of digits is equal to (9x1)+(90x2)+(900x3)+4 = 2893 digits. If this includes spaces, there would be 999 spaces, therefore there would be 3892 keystrokes.
900 of them.